Specularity of longitudinal acoustic phonons at rough surfaces
Specularity of longitudinal acoustic phonons at rough surfaces
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DOI：
10.1103/physrevb.97.045429
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发表时间：
2018-01-26
影响因子：
3.7
通讯作者：
Sinha, Sanjiv
中科院分区：
文献类型：
--
作者：
Gelda, Dhruv;Ghossoub, Marc G.;Sinha, Sanjiv
声子在晶体表面的镜面性对纳米结构中的热输运和纳米机械谐振器中的耗散具有直接的重要性。波散射理论为估计与波长相关的镜面反射率提供了一个框架，但实验验证仍然难以捉摸。广泛获得的导热系数数据的有效性很差，因为在热输运中涉及无限多的声子波长，这对镜面反射理论提出了一个欠约束的检验。在这里，我们通过测量室温下超薄(36-205 nm)悬浮硅膜中单个相干纵声声子模的寿命来报告声子镜面效应。在大于或类似于60 GHz的频率下，声子表面散射主导本征Akhiezer衰减，从而能够在类似于72-140 nm的波长上测量声子边界散射时间。利用HRTEM成像技术，我们得到了膜表面粗糙度的详细统计数据。我们发现，只有在对透射电子显微镜的统计数据进行投影误差校正后，激发模的镜面反射率才能与波散射解很好地吻合。经常被引用的齐曼声子镜面反射公式似乎也与数据很一致，与之前的结果相矛盾。这项工作有助于加深对纳米结构表面声子散射的基本理解。
The specularity of phonons at crystal surfaces is of direct importance to thermal transport in nanostructures and to dissipation in nanomechanical resonators. Wave scattering theory provides a framework for estimating wavelength-dependent specularity, but experimental validation remains elusive. Widely available thermal conductivity data presents poor validation since the involvement of the infinitude of phonon wavelengths in thermal transport presents an underconstrained test for specularity theory. Here, we report phonon specularity by measuring the lifetimes of individual coherent longitudinal acoustic phonon modes excited in ultrathin (36-205 nm) suspended silicon membranes at room temperature over the frequency range similar to 20-118 GHz. Phonon surface scattering dominates intrinsic Akhiezer damping at frequencies greater than or similar to 60 GHz, enabling measurements of phonon boundary scattering time over wavelengths similar to 72-140 nm. We obtain detailed statistics of the surface roughness at the top and bottom surfaces of membranes using HRTEM imaging. We find that the specularity of the excited modes are in good agreement with solutions of wave scattering only when the TEM statistics are corrected for projection errors. The often-cited Ziman formula for phonon specularity also appears in good agreement with the data, contradicting previous results. This work helps to advance the fundamental understanding of phonon scattering at the surfaces of nanostructures.